Charged Particle Beam Apparatus and Sample Observation Method

ABSTRACT

In the case of an in situ observation with a charged particle beam apparatus, an observer who is not an expert in the charged particle beam apparatus needs to maintain the field of view of the observation that changes from moment to moment while watching a monitor, and thus, adjustment of the field of view needs to be controllable in real time with a good operability. In order to eliminate the need for an observer to move the line of sight, a live image and a comparison image are overlapped and displayed. At this time, an interface is devised, such that overlapping of two images can be executed without giving stress to the observer. The observer presses a button on an operation screen, thereby displaying a superimposed image, which is obtained by making the comparison image matching the size of a first display area configured to display the live image translucent and superimposing the translucent comparison image on the live image, at the position of the first display area of the image display device.

TECHNICAL FIELD

The present invention relates to a charged particle beam apparatus and asample observation method using the same.

BACKGROUND ART

A charged particle beam apparatus represented by a scanning electronmicroscope scans a desired region (field of view) on a sample with acharged particle beam and records charged particle signals emitted fromthe scanned region in correspondence with scanning positions, therebyimaging an observation point. PTL 1 discloses a technique for correctingan electron beam irradiation point by using pattern matching in order tomaintain an electron beam irradiation area constant in the case ofcontinuously obtaining images or accumulating analysis result data for along time.

On the other hand, although not a technique related to a chargedparticle beam apparatus, PTL 2 discloses an image processing techniquefor synthesizing still image data with moving picture data displayed inreal time and displaying a result of the synthesis. In order to capturesymmetrical face images more precisely, when capturing a left-faceimage, a right-face image captured in advance is horizontally invertedand synthesized with a left-face image displayed in real time and aresult of the synthesis is displayed. Face images are captured byadjusting the angle of a face such that the left-face image displayed inreal time matches as much as possible with the right-face image which ishorizontally inverted, thereby capturing a symmetrical face image moreprecisely.

CITATION LIST Patent Literature

PTL 1: JP-A-2016-91893

PTL 2: JP-A-2009-61178

SUMMARY OF INVENTION Technical Problem

When a sample for observation is not changed with the lapse of time orwhen a change of the sample is caused by an operation of an observer asin PTL 1, sufficient time can be used to adjust the field of view, andthus precision of the adjustment of the field of view can be emphasizedand positional deviations of obtained image data can be digitized andcontrolled by using various image processing techniques. On the otherhand, there is a growing need for a charged particle beam apparatus tomake in situ observation of microscopic changes caused by adding heat orapplying mechanical force to a sample. In such a case, an observer whois not an expert in the charged particle beam apparatus needs tomaintain the field of view of the observation that changes from momentto moment while watching a monitor, and thus adjustment of the field ofview needs to be controllable in real time with good operability.

Adjustment of the field of view is performed by comparing a referencecomparison image and a live image of an observation. However, since themovement of the field of view becomes more significant at observationsat high magnifications, it is considered effective to overlap anddisplay two images in order to eliminate the need for an observer tomove the line of sight. To this end, it is desirable to apply the imageprocessing technique as disclosed in PTL 2. However, PTL 2 relates to anapplication field in which image capturing conditions are allowed to beadjusted over sufficient time and an object to be captured is also arelatively large object, that is, a face. In contrast, in the case ofobserving a sample in situ, an observer tries to observe changesoccurring in the field of view. A live image and a comparison image,which are displayed on a screen with sizes corresponding to an objectfor observation, are not necessarily displayed on a screen with the samedisplay size. Even in such a case, the present invention provides ascanning electron microscope and a sample observation method enabling anobserver to focus on an in-situ observation while adjusting the field ofview with a simple operation by devising an interface to overlap twoimages without giving stress to the observer.

Solution to Problem

There is provided a charged particle beam apparatus including: a chargedparticle optical system including a charged particle beam source, afocusing lens configured to focus a primary charged particle beamemitted from the charged particle beam source, an objective lensconfigured to focus the primary charged particle beam onto a sample, adeflector coil configured to scan the sample with the primary chargedparticle beam, and a detector configured to detect secondary chargedparticles generated by the irradiation of the primary charged particlebeam on the sample; an operation panel configured to receive controls ofan operator; an image display device including a plurality of displayareas; a control device connected to the operation panel and the imagedisplay device and including a device controller and a displaycontroller; and a storage device configured to store image data capturedby the charged particle optical system, wherein the device controllerreceives a control from the operation panel, controls the chargedparticle optical system, and obtains image data, the display controllerdisplays a live image obtained from the charged particle optical systemin a first display area of the image display device and displays acomparison image stored in the storage device in a second display areaof the image display device, and the display controller receives acontrol from the operation panel and displays a superimposed image,which is obtained by making the comparison image matching the size ofthe first display area translucent and superimposing the translucentcomparison image on the live image, at the position of the first displayarea of the image display device.

Advantageous Effects of Invention

The field of view can be adjusted without moving the line of sight froma live image on an image display device.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a scanning electron microscopeaccording to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of an operation screen.

FIG. 3 is a diagram showing an example of an operation screen.

FIG. 4 is a diagram showing an example of an operation screen.

FIG. 5 is a diagram showing data processing according to the embodimentof the present invention.

FIG. 6 is a flowchart of a process for overlaying a live image and acomparison image according to the embodiment of the present invention.

FIG. 7 is a diagram showing an example of a transparency setting screen.

FIG. 8 is a diagram showing an example of changing the transparency of apart of a comparison image.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described below withreference to the drawings. In the embodiments described below, ascanning electron microscope (SEM) will be described as an example of acharged particle beam apparatus, but the present invention is notlimited thereto. For example, the present invention may also be appliedto a focused ion beam (FIB) device that uses a liquid metal ion sourceor a gas ion source as a charged particle beam source and irradiates anion beam (charged particle beam) emitted therefrom onto a sample, atransmission electron microscope (TEM) a scanning transmission electronmicroscope (STEM), a scanning ion microscope, a scanning probemicroscope, an optical microscope, a laser microscope, and the like.

FIG. 1 is a schematic diagram of a scanning electron microscopeaccording to an embodiment of the present invention. A primary electronbeam 2 emitted from an electron gun (charged particle beam source) 1 isonce focused by a first focusing lens 3 and then spreads. The outerperipheral portion of the spread primary electron beam is shielded by anobjective aperture 4, and thus a center portion of the beam having apredetermined diameter is transmitted. By switching the focal positionof the first focusing lens 3 and the aperture amount of the objectiveaperture 4, a beam current amount to be irradiated onto a sample 11 canbe adjusted. A primary electron beam that passed through the objectiveaperture 4 is focused by a second focusing lens 5 and an objective lens6 onto the sample 11 placed on a sample stage 8. The primary electronbeam is scanned on the sample 11 by the action of a deflector coil 7.Signal electrons (secondary electrons) generated from the sample 11 bythe irradiation of the primary electron beam are detected by thedetector 9.

This electron optical system is arranged in a lens barrel 20 that isvacuum-exhausted and the sample stage 8 is arranged in a sample chamber21 that is vacuum-exhausted. Also, the hardware of the SEM is controlledby a control device 31. The control device 31 may be configured ashardware dedicated to processing or may be configured as software to beexecuted by using a general-purpose processor (e.g., a CPU, a GPU, aDSP, etc.). Furthermore, an image display device 32, a storage device 33and an operation panel 34 are connected to the control device 31. On theimage display device 32, an image under observation with an SEM or anoperation screen necessary for an observer to control the SEM isdisplayed as described later. The storage device 33 stores an SEMcontrol program as well as images (including a still image and a movingpicture) taken with the SEM. The operation panel 34 is an interface forinputting an instruction of an observer and is implemented as hardware,such as a keyboard, a mouse, and a pointer. For example, while viewingan image displayed on the image display device 32, the observer movesthe sample stage 8 in XYZ directions (X and Y denote two axes in a planevertical to the optical axis direction of a primary electron beam, and Zdenotes an axis along the optical axis direction of the primary electronbeam) to search for a desired observation point.

Here, in the example shown in FIG. 1, the sample 11 is placed on thesample stage 8 via a sub-stage 10. The sub-stage 10 is a stage thatapplies a predetermined action to a sample, for example, heating thesample or applying a force to the sample. When observing microscopicchanges occurring in the sample 11 by applying some actions to thesample 11 (referred to as “in-situ observation”), a sub-stage capable ofapplying a predetermined action to the sample 11 is prepared. In orderto load a sub-stage into the sample chamber 21, a relatively large inletis required in the sample chamber 21. Also, several holes are providedin the sample chamber 21. The holes are normally closed. However, incaseof using the sub-stage 10, wires required for controlling the sub-stage10 are drawn out from the holes. The drawn-out wires are connected to apower supply and a control unit in order to operate the sub-stage 10.The control of the sub-stage 10 is usually performed by a separatecontrol device (a PC or the like) different from the control device forthe SEM, but the same control device may be used.

The sample chamber 21 is connected to the lens barrel 20 via an opening22 through which a primary electron beam passes. Therefore, when thevacuum degree of the sample chamber 21 decreases, the vacuum degree ofthe lens barrel 20 also decreases. Particularly, since there are membersrequiring a high vacuum degree, such as the electron gun 1 and theobjective aperture 4, in the lens barrel 20, observation cannot bestarted unless the vacuum degree reaches a predetermined level again. Inorder to reduce the time from introduction of a sample to initiation ofobservation, it is desirable to reduce the influence of the decrease inthe vacuum degree of the sample chamber 21 to the lens barrel 20.

Therefore, a mechanism for suppressing the decrease of the vacuum degreeof the sample chamber 21 from being transmitted to the lens barrel 20side is provided. For example, by arranging a differential exhaustthrottle (not shown) for suppressing the decrease in the vacuum degreein a region of the lens barrel 20 where members 1 and 4 requiring aparticularly high vacuum degree or by using a Schottky electron gun asthe electron gun 1, the time from the opening of the sample chamber 21to the initiation of observation after another vacuum exhaustion can bereduced.

For example, an example of observing a change of the sample 11 whileheating the sample 11 will be described. The sample chamber 21 isopened, and the sample heating sub-stage 10 with the sample 11 setthereon is set on the sample stage 8. Thereafter, the sample chamber 21is vacuum-exhausted. After vacuum exhaustion is completed, an electronbeam is irradiated onto the sample 11, and an image of the currentobservation is displayed on the image display device 32. FIG. 2 shows anexample of an operation screen 51 displayed on the image display device32.

The operation screen 51 includes a live image display area 52, acaptured image display area 53, and an operation panel 55. Variousparameters for controlling the SEM for observation are displayed on theoperation panel 55, and an observer performs parameter adjustment viathe operation panel 34. Since parameters for capturing appropriateimages under predetermined conditions are stored in advance in thestorage device 33, appropriate parameters may be read out from thestorage device 33 and the observer may not need to change thepredetermined parameters in most cases. In the live image display area52, an image of the current observation with the SEM is displayed.

Also, when the observer captures an image during the observation,corresponding image data is stored in the storage device 33. Inaddition, a reduced image 54-i (i is an integer) of the captured imageis displayed in the captured image display area 53. All of a series ofimages captured during a single observation are displayed in thecaptured image display area 53. Here, images may not only be stillimages, but also be moving pictures. In the case of a moving picture,for example, an initial image is displayed as a representative imagethereof.

It is assumed that the observer observes changes of the sample 11 whilegradually increasing the temperature of the sample 11 from roomtemperature to a target temperature A and to a target temperature B.First, the sample stage 8 is moved at room temperature and a field ofview for observation is decided. The field of view for observationincludes markers that do not change their positions and shapes even ifthe temperature rises, such that the observer maintains the same fieldof view. For example, a foreign object included in the field of view canbe used as a marker. When the field of view is decided, an image thereofis captured. A reduced captured image 54 is displayed in the capturedimage display area 53.

For example, when a reduced image 54-1 is double-clicked, the reducedimage 54-1 is displayed in a comparison image display area 56. Thecomparison image display area 56 is a window different from areas 52,53, and 55 and displays the captured image 54-1 as an image of the samesize and the same magnification as in the live image display area 52.Since images are opened in different windows, the comparison imagedisplay area 56 can be displayed at any position on the operation screen51. The live image display area 52 and the comparison image display area56 can be displayed with different display sizes. This is because it isdesirable that a target of interest of an observer is displayed on anoperation screen with an appropriate size.

When the target temperature A is set and the heating of the sample 11 isstarted, the temperature of the sample 11 starts to rise toward thetarget temperature A. As the temperature of the sample 11 rises, theperiphery of the sample 11 also is warmed up and thermally expands as awhole, and thus the field of view continues to gradually move from theobservation position determined at a room temperature. Therefore, it isnecessary to operate the sample stage 8 so as not to lose the field ofview. When the live image display area 52 and the comparison imagedisplay area 56 are arranged adjacent to each other as shown in FIG. 3,comparison of two images may be facilitated. However, for example, it isdifficult to determine whether a foreign object image 102 and a foreignobject image 103, which are markers of the field of view, are at thesame positions in a live image 100 and a comparison image 101. Sincethere is a case where a live image is changed instantaneously due to atemperature change, if an observer happens to move the line of sight toa comparison image display area at such a timing, the target field ofview may be lost.

Therefore, in the present embodiment, a button 57 is provided at theupper portion of a window displaying the comparison image 101. When thebutton 57 is clicked, a superimposed image 104, which is obtained bymaking the comparison image 101 translucent and superimposing the sameon the live image 100, is displayed as illustrated in FIG. 4. As imagesare overlapped, since it is not necessary to move the line of sight fromthe live image 100 for adjusting the field of view and the comparisonimage 101 is translucent, it is easy to determine whether a marker is amarker of the live image 100 or a marker of the comparison image 101.

When the temperature of the sample 11 reaches the set target temperatureA and stabilizes, movement of the field of view is stopped. The field ofview is finely adjusted by operating the sample stage 8 and an image iscaptured. A reduced image of the captured image is displayed in thecaptured image display area 53. When the button 57 is clicked again inthe state where images are overlapped, the operation screen 51 returnsto the state shown in FIG. 3. In other words, the comparison image 101returns to its original transparency and is displayed in a differentwindow at a position different from that in the live image.Alternatively, the comparison image 101 may be removed from theoperation screen 51.

Next, when the target temperature B is set, the temperature of thesample 11 starts to rise toward the target temperature B. The observermay use an image captured at room temperature or an image captured atthe target temperature A as a comparison image. Since any of capturedimages is displayed in the captured image display area 53, it is easy tomake a selection. Thereafter, the observation can be continued in thesame regard.

In addition, incase of applying a tension force to the sample 11 andobserving changes thereof, a sub-stage for pulling the sample 11 isprepared. When the sample 11 is pulled in two opposite directions anddeformed, the field of view except the center portion moves. In such acase, by using the operation screen according to the present embodiment,the field of view can be adjusted without moving the line of sight fromthe live image 100.

A processing flow for overlapping images will be described withreference to FIGS. 5 and 6. Signal electrons detected by a detector 9are converted into image data DL (P, X, Y) (1≤X≤L, 1≤Y≤M) in the controldevice 31. Image data is expressed as a set of pixel values P at theposition (X, Y) in an image. In order to display the image data DL onthe image display device 32, the image data DL is converted into displayimage data EL (p, x, y) (1≤x≤j, 1≤y≤k) matching the size of the liveimage display area 52 and is displayed as the live image 100 in the liveimage display area 52 of the operation screen 51. On the other hand,when the observer captures an image, image data obtained from thedetector 9 is stored in the storage device 33 as DSi (P, X, Y) (1≤i≤N,1≤X≤L, 1≤Y≤M). N denotes the number of images taken by the observer.

A captured image is converted into reduced display image data ETSi (p,x′, y′) (1≤x′≤j′, 1≤y′≤k′) matching the size of the captured imagedisplay area 53 and, as described above, is displayed in the capturedimage display area 53 of the operation screen 51 as the reduced image54. When the reduced image 54 is double-clicked (assuming that a reducedimage of i=n is double-clicked), the display image data ESn (p, x, y)(1≤x≤j, 1≤y≤k) matching the size of the comparison image display area 56and is displayed in the comparison image display area 56 of theoperation screen 51 as the comparison image 101.

By detecting a click of the button 57 in this state (see FIG. 3), adisplay controller 36 starts processing (S70). Here, a processing startcommand may be input, for example, by a predetermined key manipulationof the operation panel 34. First, it is checked whether a live image anda comparison image are of the same size (S72). In the example shown inFIG. 5, the sizes of the live image and the comparison image are thesame. However, when the size of a live image is different from that of acomparison image, pixel data of the comparison image at the size of thelive image is obtained (S74). Next, the representative coordinates ofthe live image on the operation screen 51 is obtained (S76).

For example, as shown in FIG. 2, with a pixel 60 at the upper leftcorner of the operation screen 51 as an origin, the coordinates of anarbitrary pixel Q on the operation screen 51 are expressed as Q(a, b)and the upper left corner of the display area is used as therepresentative coordinate, the coordinate QL(al, bl) of a pixel 61 isobtained as the representative coordinate of the live image displayarea. Here, the method of assigning the coordinate of the operationscreen 51 and the method of assigning the representative image of thedisplay area are not limited thereto, and any method may be used as longas the coordinate is identifiable. In addition, when the live imagedisplay area 52 is fixed on the operation screen 51, this step can beomitted.

Next, a superimposed image obtained by superimposing a comparison imagewith the same size and a predetermined opaqueness to a live image isgenerated (S78). For example, alpha blending technique is applied tosuperimposing two images. In the alpha blending technique, as a pixelvalue, each pixel includes an alpha value a indicating transparency. Asa result, by mixing colors of pixels of the live image and colors ofpixels of the comparison image at ratios corresponding to the alphavalues a, a superimposed image in which the translucent comparison imageis superimposed on the live image can be generated. The generatedsuperposed image is displayed in the comparison image display area 56and the comparison image display area 56 is displayed at the position ofthe representative coordinate of the live image (S80). This can berealized by setting the value of the coordinate QC (ac, bc) of a pixel62, which is the representative coordinate of the comparison imagedisplay region, to the coordinate (al, al).

Here, the superimposed image may also be displayed in the live imagedisplay area 52.

In this case, the comparison image display area 56 is not moved. Sincethe superimposed image is displayed in the live image display area 52,the same effect can be obtained.

Furthermore, as shown in FIG. 7, in order to adjust the translucenttransparency, a transparency adjustment tool 58 is displayed when themouse is moved over the button 57. Transparency may be adjusted by aslider 59 or by directly inputting a numerical value. At this time, thetransparency may be input as a ratio or a value of the alpha value a (0to 255) may be directly input. The transparency set here is stored inthe storage device 33.

In the present embodiment, two images are overlapped for the purpose ofadjusting the field of view of a live image, but various methods may beconsidered for image processing. For example, contour lines may beextracted from a comparison image to be overlapped on a live image.However, the method of the present embodiment in which a comparisonimage is translucent and superimposed on a live image is consideredadvantageous in the following points. First, since only values of pixelsto be displayed on the image display device 32 need to be calculated,the processing load is small, and thus it is possible to easily followthe operation of the observer. In addition, if a foreign object is usedas a marker, it may be difficult to align images without extractingcontour lines at a certain precision.

Although the present embodiment has been described above in detail,various modifications can be made. For example, a plurality ofcomparison image display areas 56 can be displayed on the operationscreen 51, and the button 57 is provided in each of the comparison imagedisplay areas 56. As the buttons 57 of any one of the comparison imagedisplay areas 56 is clicked, a comparison image displayed in thecorresponding comparison image display area 56 is made translucent, anda superimposed image obtained by superimposing the translucentcomparison image on a live image is displayed. Furthermore, in thisstate, when the button 57 of another comparison image display area 56 isclicked, a comparison image displayed in the comparison image displayarea 56 where the button 57 is clicked is made translucent and thesuperimposed image obtained by superimposing the translucent comparisonimage on a live image is replaced.

In case of displaying a plurality of comparison images, it is preferableto use a common transparency value for the set transparency value. Inother words, transparency set to one comparison image is also reflectedin other comparison images. Since many of images taken by onemeasurement are similar to one another, it is convenient for an observerto reflect one set value to all comparison images instead of storingdifferent set values for respective comparison images. Therefore, forexample, the latest transparency is set for a plurality of capturedimages displayed in the captured image display area 53. Furthermore,when the brightness differs from one image to another in onemeasurement, one transparency value may be set for one comparison imageand, after the transparency value is reflected in all comparison images,the observer may set different transparency.

In order to simultaneously display images of various different signalsas live images on the operation screen 51, images may be displayed inmultiple screens, such as two screens or four screens, in the live imagedisplay area 52. Since a screen for manually adjusting the brightnessand the contrast of an image needs to be limited to one screen, even inthe case of the multi-screen display, only one screen is selected. Whenthe live image display area 52 displays images in multi-screens and thebutton 57 of the comparison image display area 56 is pressed, acomparison image is superimposed on a live image whose brightness andcontrast can be adjusted, that is, a selected live image.

Furthermore, as shown in FIG. 8, regions to be made translucent aredesignated to be regions 105 and 106 in the comparison image 101 andonly the designated regions of the comparison image 101 can be madetranslucent and superimposed whereas the other regions of the comparisonimage are not superimposed (transparency is set to 100%). As a result,there is an advantage that portions of a live image irrelevant to anadjustment of the field of view can be clearly seen.

REFERENCE SIGNS LIST

1: electron gun

2: primary electron beam

3: first focusing lens

4: objective aperture

5: second focusing lens

6: objective lens

7: deflector coil

8: sample stage

9: detector

10: sub-stage

11: sample

20: lens barrel

21: sample chamber

31: control device

32: image display device

33: storage device

34: operation panel

1. A charged particle beam apparatus comprising: a charged particleoptical system including a charged particle beam source, a focusing lensconfigured to focus a primary charged particle beam emitted from thecharged particle beam source, an objective lens configured to focus theprimary charged particle beam onto a sample, a deflector coil configuredto scan the sample with the primary charged particle beam, and adetector configured to detect secondary charged particles generated byirradiation of the primary charged particle beam on the sample; anoperation panel configured to receive controls of an operator; an imagedisplay device including a plurality of display areas; a control deviceconnected to the operation panel and the image display device andincluding a device controller and a display controller; and a storagedevice configured to store image data captured by the charged particleoptical system, wherein the device controller receives a control fromthe operation panel, controls the charged particle optical system, andobtains image data, the display controller displays a live imageobtained from the charged particle optical system in a first displayarea of the image display device and displays a comparison image storedin the storage device in a second display area of the image displaydevice, and the display controller receives a control from the operationpanel and displays a superimposed image, which is obtained by making thecomparison image matching the size of the first display area translucentand superimposing the translucent comparison image on the live image, atthe position of the first display area of the image display device. 2.The charged particle beam apparatus according to claim 1, wherein thedisplay controller generates image data of the superimposed image bysynthesizing image data of the live image which is set to firsttransparency with image data of the comparison image which is set tosecond transparency different from the first transparency.
 3. Thecharged particle beam apparatus according to claim 2, wherein thedisplay controller displays a button in the second display area anddisplays the superimposed image at the position of the first displayarea of the image display device when the displayed button receives afirst control from the operation panel.
 4. The charged particle beamapparatus according to claim 3, wherein the second display area, inwhich the superimposed image is displayed, is displayed at the positionof the first display area of the image display device.
 5. The chargedparticle beam apparatus according to claim 3, wherein the displaycontroller displays the superimposed image in the first display area. 6.The charged particle beam apparatus according to claim 3, wherein thedisplay controller displays a transparency adjustment tool for adjustingthe transparency as the button receives a second control from theoperation panel.
 7. The charged particle beam apparatus according toclaim 2, wherein the display controller displays a plurality of capturedimages, which are stored in the storage device, in a third display areaof the image display device as reduced images, the display controllerdisplays one of the plurality of captured images, which is selected bythe operation panel, in the second display area as the comparison image,and the transparency is set to a common value for the plurality ofcaptured images.
 8. A sample observation method using a charged particlebeam apparatus comprising an operation panel configured to receive anoperator's control, an image display device including a plurality ofdisplay areas, a sample stage, a charged particle optical system, and astorage device configured to store image data captured by the chargedparticle optical system, the method comprising: placing a sample on thesample stage via a sub-stage; displaying a live image obtained from thecharged particle optical system in a first display area of the imagedisplay device, displaying a comparison image stored in the storagedevice in a second display area of the image display device; controllingthe sub-stage to apply a predetermined action to the sample; and,displaying a superimposed image, which is obtained by making thecomparison image matching the size of the first display area translucentand superimposing the translucent comparison image on the live image, atthe position of the first display area of the image display device as acontrol is received from the operation panel.
 9. The sample observationmethod according to claim 8, wherein image data of the superimposedimage is generated by synthesizing image data of the live image which isset to first transparency with image data of the comparison image whichis set to second transparency different from the first transparency. 10.The sample observation method according to claim 9, wherein a button isdisplayed in the second display area and the superimposed image isdisplayed at the position of the first display area of the image displaydevice when the displayed button receives a first control from theoperation panel.
 11. The sample observation method according to claim10, wherein a transparency adjustment tool for adjusting thetransparency is displayed as the button receives a second control fromthe operation panel.
 12. The sample observation method according toclaim 9, wherein a plurality of captured images, which are stored in thestorage device, are displayed in a third display area of the imagedisplay device as reduced images, one of the plurality of capturedimages, which is selected by the operation panel, is displayed in thesecond display area as the comparison image, and the transparency is setto a common value for the plurality of captured images.